Process for the production of a light gasoline with a low sulphur content

ABSTRACT

A process for the treatment of a gasoline comprising diolefins, olefins and sulphur-containing compounds including mercaptans:
     a) demercaptanization by addition of at least a portion of the mercaptans onto the olefins by bringing the gasoline into contact with at least one first catalyst;   b) treatment of the gasoline obtained from a) with hydrogen in a distillation column comprising at least one reaction zone including at least one second catalyst. The operating conditions and the second catalyst of b) are selected such that in the distillation column, separation of the gasoline obtained from a) into a light gasoline fraction and a heavy gasoline fraction is carried out simultaneously with a reaction for thioetherification and selective hydrogenation of the diolefins of a gasoline fraction obtained from a) by contact with the second catalyst.

The present invention relates to a process for the treatment of a gasoline comprising diolefins, olefins and sulphur-containing compounds, including mercaptans, with a view to providing a light fraction of this gasoline with a very low sulphur content while preserving the octane number.

PRIOR ART

The production of reformulated gasolines complying with new environmental standards in particular requires that their concentration of olefins be reduced slightly, but that their concentration of aromatics (in particular benzene) and sulphur be substantially reduced. Catalytically cracked gasolines, which may represent 30% to 50% of the gasoline pool, have high olefins and sulphur contents. Close to 90% of the sulphur present in reformulated gasolines can be attributed to gasoline from catalytic cracking (FCC, Fluid Catalytic Cracking). Desulphurization (hydrodesulphurization) of gasolines, principally FCC gasolines, is thus of substantial importance when complying with the specifications.

Pre-treatment by hydrotreatment (hydrodesulphurization) of feeds sent for catalytic cracking results in FCC gasolines typically containing less than 100 ppm of sulphur. These hydrotreatment units, however, operate under severe temperature and pressure conditions, which presupposes a high hydrogen consumption and high costs. In addition, the whole of the feed has to be desulphurized, which involves the treatment of very large volumes of feed.

Thus, in order to satisfy specifications as regards sulphur, it is necessary to post-treat the catalytically cracked gasolines by hydrotreatment (or hydrodesulphurization). When this post-treatment is carried out under conventional conditions known to the skilled person, it is possible to further reduce the sulphur content of the gasoline. However, this process suffers from the major disadvantage of causing a very large drop in the octane number of the cut, due to saturation of olefins during the hydrotreatment.

U.S. Pat. No. 4,131,537 discloses the advantage of fractionating the gasoline into several cuts, preferably three, as a function of their boiling point, and of desulphurizing them under conditions which may be different and in the presence of a catalyst comprising at least one metal from group VIB and/or group VIII. That patent points out that the greatest benefit is obtained when the gasoline is fractionated into three cuts, and when the cut with intermediate boiling points is treated under mild conditions.

French Patent FR 2 785 908 discloses the advantage of fractionating the gasoline into a light fraction and a heavy fraction then carrying out a specific hydrotreatment of the light gasoline over a nickel-based catalyst and a hydrotreatment of the heavy gasoline over a catalyst comprising at least one metal from group VIII and/or at least one metal from group VIb.

American patent U.S. Pat. No. 6,440,299 describes a process for the elimination of mercaptans from a hydrocarbon feed using a catalytic distillation column. The catalytic bed of the column is located above the inlet so that only the light fraction of the feed is treated. The catalyst used is a supported catalyst based on nickel sulphide over which mercaptans are eliminated by means of a thioetherification reaction by addition onto diolefins. As illustrated in the example of that patent, in that process it is difficult to simultaneously obtain very low sulphur contents for the light fraction of the treated gasoline. In fact, when the quantity of diolefins in the feed is low and/or the quantity of mercaptans is high, the conversion kinetics for the mercaptans on the catalyst is not favourable. In order to maintain a high conversion, either the temperature has to be increased, or internal traffic in the column has to be limited. Operating at a higher temperature with the same cut point for the light gasoline can only be carried out by increasing the pressure in the column, but that increase is limited by the design of the column. Limiting the internal traffic (for example by reducing the internal reflux ratio) suffers from the disadvantage of degrading the separating power of the column, which would promote the recovery of unconverted light mercaptans in the light fraction.

U.S. Pat. No. 5,807,477 is also known, and discloses a process for the treatment of a catalytically cracked gasoline; it uses a first step for thioetherification in which the mercaptans are reacted with the diolefins of the feed in the presence of a catalyst based on an oxide of a metal from group VIII. The gasoline which has undergone this first step is then sent to a distillation column comprising a catalytic hydrogenation section containing another catalyst based on an oxide of a metal from group VIII. The catalytic column is configured so as to function as a depentanizer. Thus, compounds containing 6 or more carbon atoms and including the thioethers formed during the first step are withdrawn from the bottom of the column, while compounds containing 5 or fewer carbon atoms are distilled towards the head of the column and brought into contact with the hydrogenation catalyst where selective hydrogenation of the diolefins which have not reacted during the first step is carried out.

However, in the process described above, it is difficult to obtain a light fraction of gasoline which complies with future specifications as regards sulphur, i.e. with an upper limit for the total sulphur in the gasoline of 50 ppm by weight, or even 30 or 10 ppm by weight in some countries.

As indicated in the example in patent U.S. Pat. No. 5,807,477, the first step of the process converts few or no light mercaptans. The mercaptans are in fact eliminated by addition onto the diolefins in the feed. However, that patent specifies that such a catalyst based on an oxide of a metal from group VIII also catalyses the selective hydrogenation of the diolefins. Thus, the two reactions involving the diolefins are concurrent, resulting in a limited conversion as regards thioetherification. Once injected into the column (second step), the light mercaptans, more particularly those which are lighter than the cut point of the column, will preferentially be in the vapour phase in the rectification zone, and thus it will be difficult to convert them over the catalytic bed. The light gasoline produced at the head of the catalytic column thus contains a large fraction of the light mercaptans present in the initial feed.

Similarly, in the example mentioned in that patent, it will be observed that H₂S is not converted at all in that first step, which means that if H₂S is present in the starting feed, it will inevitably be found in the catalytic column located downstream. However, the skilled person is aware that hydrogenation catalysts based on a metal from group VIII also catalyse reactions for the addition of H₂S onto diolefins and/or olefins, which could induce the formation of additional mercaptans close to the head of the column, which are primarily found in the light fraction of the gasoline. Furthermore, H₂S and mercaptans are known to be inhibitors of this type of catalyst, in particular when the metal from group VIII is palladium.

Thus, it is difficult for the invention as presented in U.S. Pat. No. 5,807,477 to reduce the sulphur content of the light fraction of the gasoline to very low levels, and so in general, a post-treatment of this fraction is necessary in order for it to comply with specifications.

Thus, one aim of the invention is to propose a process for the production of a light gasoline with a very low sulphur content, i.e. with a sulphur content of less than 50 ppm by weight, preferably less than 30 ppm or 10 ppm by weight, while limiting the drop in octane number, which is also relatively simple and which requires as small an investment as is possible.

SUMMARY OF THE INVENTION

To this end, a process for the treatment of a gasoline comprising diolefins, olefins and sulphur-containing compounds including mercaptans is proposed, said process comprising the following steps:

a) carrying out a step for demercaptanization by addition of at least a portion of the mercaptans onto the olefins by bringing gasoline into contact with at least one first catalyst, at a temperature in the range 50° C. to 250° C., at a pressure in the range 0.4 to 5 MPa and with a liquid hourly space velocity (LHSV) in the range 0.5 to 10 h⁻¹, the first catalyst being in the sulphide form and comprising a first support, at least one metal selected from group VIII and at least one metal selected from group VIb of the periodic classification of the elements, the % by weight, expressed as the oxide equivalent of the metal selected from group VIII with respect to the total catalyst weight, being in the range 1% to 30% by weight and the % by weight, expressed as the oxide equivalent of the metal selected from group VIb, being in the range 1% to 30% with respect to the total catalyst weight;

b) carrying out a step for treating the gasoline obtained from step a) with hydrogen in a distillation column comprising at least one reaction zone comprising at least one second catalyst comprising a second support and at least one metal from group VIII, the conditions of step b) being selected such that the following operations are carried out simultaneously in said distillation column:

I) distillation in order to separate the gasoline obtained from step a) into a light gasoline fraction which is depleted in sulphur-containing compounds and into a heavy gasoline fraction with a boiling temperature which is higher than that of the light gasoline and comprising the majority of the sulphur-containing compounds, the light gasoline fraction being evacuated at a point located above the reaction zone and the heavy gasoline fraction being evacuated at a point below the reaction zone;

II) bringing a fraction of the gasoline obtained from step a) into contact with the second catalyst in order to carry out the following reactions:

i) thioetherification, by addition of a portion of the mercaptans onto a portion of the diolefins to form thioethers;

ii) selective hydrogenation of a portion of the diolefins to olefins; and optionally

iii) isomerization of the olefins.

The process of the invention employs a first step a) in which the sulphur-containing mercaptan type compounds (R—SH) are transformed into heavier sulphur-containing compounds by reaction with the olefins present in the gasoline to be treated. The demercaptanization reactions of the invention are characterized by reacting the mercaptans with the olefins:

-   -   either by direct addition onto the double bond to produce         sulphides with a higher boiling point;     -   or by a hydrogenolysing pathway: the hydrogen present in the         reactor will produce H₂S by contact with a mercaptan which will         add directly onto the double bond of an olefin to form a heavier         mercaptan, i.e. with a higher boiling point. However, this         pathway is a minor pathway under the preferred conditions of the         reaction.

This first step of weighting of mercaptans reaches very high conversions (>90% and very often >95%), as the demercaptanization reactions occur selectively on the olefins which are generally present in high quantities. The lightest mercaptans are the most reactive in this first step a).

Similarly, if H₂S is present in the feed, it is converted into mercaptan (which can itself be converted) by addition onto the olefins by means of a catalyst under selected conditions. Eliminating the H₂S right from this first step is advantageous in that it means that inhibition of the catalyst for the catalytic column for the second step can be prevented (H₂S is an inhibitor for the catalyst used) and entrainment of H₂S at the head with the light fraction can be prevented. The conversion of the H₂S upstream of the distillation column into heavy mercaptans or sulphides which will leave in the heavy fraction from the catalytic distillation column means that the sulphur content in the light fraction is very low. Thus, 100% of the H₂S present in a quantity of up to of the order of 10 ppm by weight in the feed can be converted.

The demercaptanization and H₂S elimination reactions are preferably carried out over a catalyst comprising at least one metal from group VIII (groups 8, 9 and 10 of the new periodic classification of the elements, Handbook of Chemistry and Physics, 76^(th) edition, 1995-1996), at least one metal from group VIb (group 6 of the new periodic classification of the elements, Handbook of Chemistry and Physics, 76^(th) edition, 1995-1996) and a support. The metal from group VIII is preferably selected from nickel and cobalt, and in particular nickel. The metal from group VIb is preferably selected from molybdenum and tungsten; highly preferably, it is molybdenum.

Before bringing it into contact with the feed to be treated, the catalyst undergoes a sulphurization step. The catalyst only brings about the desired demercaptanization reactions when it is in its sulphide form. Sulphurization is preferably carried out in a sulphoreducing medium, i.e. in the presence of H₂S and hydrogen, in order to transform the metallic oxides into sulphides such as, for example, MoS₂ and Ni₃S₂.

The process of the invention comprises a second step b) for treating the gasoline produced in step a) in a distillation column provided with a reaction section.

The distillation column is configured to operate under operating conditions which can simultaneously:

-   -   transform the light mercaptans which have not reacted during         step a) by thioetherification with diolefins so that the         thioethers formed during the second step are also entrained in         the heavy gasoline fraction. The catalyst used in the         distillation column also carries out selective hydrogenation of         the diolefins and can possibly isomerize olefins where the         double bond is in an external position to those with an internal         position;     -   separate the gasoline pre-treated in step a) into at least two         fractions, namely a gasoline fraction termed a “light” fraction         which is depleted in sulphur-containing compounds and which         contains the major portion of the olefins, and a gasoline         fraction termed the “heavy” fraction which comprises the         majority of the sulphur-containing compounds, in particular the         thioethers formed during steps a) and b).

The reaction for the selective hydrogenation of diolefins to olefins is especially important when the light cut of the gasoline is used as a feed for an etherification unit (of the TAME type, for example), because these highly unsaturated compounds readily form gums in this type of process. When this light cut is sent directly to the gasoline pool, it is also advantageous to hydrogenate the diolefins, as these latter have a tendency to produce gums when oxygen enters the storage tanks. In addition, this second step can also be used to weight the residual mercaptans which have not been transformed during step a), by reaction of the residual mercaptans with the diolefins. Finally, depending on what metal from group VIII is selected for one of the supported catalysts of the column, preferably palladium, the invention may propose isomerization of the olefins from the external position to the internal position. This isomerization is advantageous, as it means that the octane number of the cut can be maintained or even improved, despite the hydrogenation of the olefins which may take place in each of the steps.

One advantage of the process of the invention derives from the fact that step a) does not generate hydrogen sulphide (H₂S) but, in contrast, can convert the H₂S which might be present in the feed to be treated. In fact, the presence of H₂S at the inlet to the catalytic column could, as indicated above, induce the formation of additional mercaptans on the catalytic zone of the column which could then leave in the light fraction. The absence of H₂S, which is an inhibitor of the catalysts employed in the present invention, can also be used to improve the efficiency of the catalyst for the catalytic column.

Another advantage of the process is that the large conversion of the mercaptans during step a) induces a large increase in the molar ratio between the mercaptans and diolefins at the column inlet. The efficiency of the thioetherification of the residual mercaptans in step b) is thus improved because the mercaptans/diolefins ratio is favourable.

Another advantage of the concatenation of steps a) and b) is linked to the presence of a synergistic effect between these two steps for conversion of the mercaptans due to a difference in the reactivity of the mercaptans as a function of their chain length in the two steps. In fact, the lightest mercaptans (methanethiol, ethanethiol in particular) are the most reactive in step a). In contrast, the heaviest mercaptans which could be entrained in the light cut (for example 1- and 2-propanethiol), i.e. those for which the boiling point is close to the cut point of the column, are less reactive in step a), while they are more readily converted in the catalytic column since they are favourably present in the liquid phase in the catalytic zone. The synergistic effect of the two reactions thus means that a light cut of gasoline with a very low sulphur content can be obtained irrespective of the nature of the mercaptans initially present in the feed.

Another advantage of the process of the invention resides in the fact that it is not necessary to desulphurize the light fraction of the gasoline obtained from the distillation column, because the major portion of the sulphur-containing compounds have been transformed into compounds with a higher molecular weight during steps a) and b), and so they are entrained in the heavy gasoline fraction.

The catalytic hydrogenation reactions are not required in step a), and so hydrogen, if it is added, essentially acts to maintain a hydrogenating surface condition for the catalyst so as to ensure a high yield for the demercaptanization reactions. The process of the invention is thus not penalized by the low pressures. Another advantage of the process is thus that the two steps can be carried out at the same pressure (apart from the pressure drop) as step a) only requires a little dissolved hydrogen, or even none at all, and step b) is generally carried out at relatively low pressures (less than 1.5 MPa) in order to minimize consumption of the utilities while ensuring an appropriate temperature for the thioetherification reaction. This step b) also necessitates a relatively small quantity of makeup hydrogen in order to selectively hydrogenate only the diolefins.

DETAILED DESCRIPTION OF THE INVENTION

The aim of the present invention is to provide a process for the production of a light fraction of a gasoline with a low sulphur content starting from a gasoline preferably obtained from a catalytic cracking, coking or visbreaking unit.

In accordance with the invention, the gasoline first undergoes a step a) for transformation of the sulphur-containing compounds, essentially the lightest mercaptans of the gasoline, over olefins in order to increase their molecular weight. The process also comprises a second step b) which consists of passing all or a portion of the gasoline obtained in step a) into a distillation column comprising a catalytic reaction zone. In step b), in which fractionation of the gasoline is carried out into at least two fractions (a light fraction and a heavy fraction), the light fraction is treated under conditions and over a catalyst which can enable i) the formation of thioethers by addition of a portion of the mercaptans which have not reacted in step a) with a portion of the diolefins in order to form sulphur-containing compounds with a higher boiling point which are subsequently found in the heavy gasoline, ii) selective hydrogenation of at least a portion of the diolefins to olefins, and optionally iii) isomerization of olefins with an external double bond to those with an internal double bond.

This concatenation can be used to obtain a final light fraction the sulphur content of which has been reduced without a substantial reduction of the olefins content, even for intense levels of conversion, this being without the need to treat this light gasoline using a hydrodesulphurization section or having recourse to processes which can restore the octane number of the gasoline.

Thus, the process of the invention can be used to provide a light gasoline fraction which has a total sulphur content of less than 50 ppm by weight, preferably less than 30 ppm, or even less than 10 ppm by weight.

In the context of the present application, the expression “catalytic column” designates an apparatus in which the catalytic reaction and separation of the products takes place at least simultaneously. The apparatus employed may comprise a distillation column equipped with a catalytic section in which the catalytic reaction and distillation take place simultaneously at the specific selected cut point. It may also be a distillation column in association with at least one reactor disposed inside said column and on a wall thereof. The internal reactor may be operated as a vapour phase reactor or as a liquid phase reactor, with circulation of the liquid/vapour as a co-current or as a counter-current.

Using a catalytic distillation column has the advantage over the use of a system comprising a reactor and a distillation column, of reducing the number of unitary elements and hence reducing investment costs. Using a catalytic column means that the reaction can be controlled while promoting exchange of the heat released; the heat of reaction can be absorbed by the heat of vaporization of the mixture.

The Gasoline to be Treated

The process of the invention can be used to treat any type of gasoline cut containing sulphur, preferably a gasoline cut obtained from a catalytic cracking unit, for which the boiling point range typically extends from approximately the boiling points of hydrocarbons containing 2 or 3 carbon atoms (C2 or C3) to approximately 250° C., more preferably from approximately the boiling points of hydrocarbons containing 2 or 3 carbon atoms (C2 or C3) to approximately 220° C., more preferably from approximately the boiling points of hydrocarbons containing 5 carbon atoms to approximately 220° C. The process of the invention can also be used to treat feeds with end points below those mentioned above such as, for example, a C5-180° C. cut. The sulphur content of the gasoline cuts produced by catalytic cracking (FCC) depends on the sulphur content of the feed treated by FCC, the presence or otherwise of a pre-treatment of the FCC feed, as well as the end point of the cut. In general, the sulphur contents of the whole of a gasoline cut, in particular those from FCC, are more than 100 ppm by weight, most of the time more than 500 ppm by weight. For gasolines with end points of more than 200° C., the sulphur contents are often more than 1000 ppm by weight, and may in some cases even reach values of the order of 4000 to 5000 ppm by weight.

In addition, the gasolines obtained from catalytic cracking units (FCC) contain, on average, between 0.5% and 5% by weight of diolefins, between 20% and 50% by weight of olefins, between 10 ppm and 0.5% by weight of sulphur, generally including less than 300 ppm of mercaptans. The mercaptans are generally concentrated in the light fractions of the gasoline, and more precisely in the fraction with a boiling point below 120° C.

It should be noted that the sulphur-containing compounds present in the gasoline may also comprise heterocyclic sulphur-containing compounds such as thiophenes, alkylthiophenes or benzothiophenes, for example.

Step a) for Weighting the Mercaptans with Olefins

This step consists of transforming light sulphur-containing compounds from the mercaptans family, i.e. compounds which are in the light gasoline after the distillation step b), into heavier sulphur-containing compounds which are entrained in the heavy gasoline fraction during distillation step b).

During this step a), a demercaptanization reaction occurs which is targeted at addition of mercaptans to the olefins of the feed in the presence of a catalyst.

Typically, the mercaptans which can react during step a) are as follows (non-exhaustive list): methyl mercaptan, ethyl mercaptan, n-propyl mercaptan, isopropyl mercaptan, isobutyl mercaptan, tert-butyl mercaptan, n-butyl mercaptan, sec-butyl mercaptan, isoamyl mercaptan, n-amyl mercaptan, α-methylbutyl mercaptan, α-ethylpropyl mercaptan, n-hexyl mercaptan and 2-mercaptohexane.

The demercaptanization reaction is preferably carried out over a catalyst comprising at least one metal from group VIII (groups 8, 9 and 10 of the new periodic classification of the elements, Handbook of Chemistry and Physics, 76^(th) edition, 1995-1996), at least one metal from group VIb (group 6 of the new periodic classification of the elements, Handbook of Chemistry and Physics, 76^(th) edition, 1995-1996) and a support. The metal from group VIII is preferably selected from nickel and cobalt, and in particular nickel. The metal from group VIb is preferably selected from molybdenum and tungsten, and highly preferably is molybdenum.

The support for the catalyst is preferably selected from alumina, nickel aluminate, silica, silicon carbide, or a mixture of these oxides. Preferably, alumina is used and more preferably, pure alumina. Preferably, a support with a total pore volume, measured by mercury porosimetry, in the range 0.4 to 1.4 cm³/g is used, preferably in the range 0.5 to 1.3 cm³/g. The specific surface area of the support is preferably in the range 70 m²/g to 350 m²/g. In a preferred variation, the support is a cubic gamma alumina or delta alumina.

The catalyst used in step a) generally comprises:

-   -   a quantity by weight of oxide of the metal from group VIb in the         range 1% to 30% by weight with respect to the total catalyst         weight;     -   a quantity by weight of oxide of the metal from group VIII in         the range 1% to 30% by weight with respect to the total catalyst         weight;     -   a degree of sulphurization of the constituent metals of said         catalyst of at least 60%;     -   a molar ratio between the metal from group VIII and the metal         from group VIb in the range 0.6 to 3 mol/mol;     -   a support constituted by gamma or delta alumina with a specific         surface area in the range 70 m²/g to 350 m²/g.

In particular, it has been found that the performance of the catalysts is improved when the catalyst has the following characteristics:

-   -   a quantity by weight of oxide of the metal from group VIb in the         range 4% to 20% by weight with respect to the total catalyst         weight, preferably in the range 6% to 18% by weight;     -   a quantity by weight of oxide of the metal from group VIII in         the range 3% to 15% by weight, preferably in the range 4% by         weight to 12% by weight with respect to the total catalyst         weight;     -   a molar ratio between the metal from group VIII and the metal         from group VIb in the range 0.6 to 3 mol/mol, preferably in the         range 1 to 2.5 mol/mol;     -   a support constituted by gamma alumina with a specific surface         area in the range 180 m²/g to 270 m²/g.

In a preferred embodiment of the invention, a catalyst is used containing in the range 4% to 12% by weight of nickel oxide (in the form NiO), in the range 6% to 18% by weight of molybdenum oxide (in the form MoO₃) and with a nickel/molybdenum molar ratio in the range 1 to 2.5, the metals being deposited on a support constituted solely by alumina and with a degree of sulphurization of the metals constituting the catalyst of more than 80%.

The catalyst of the invention may be prepared using any technique which is known to the skilled person, in particular by impregnation of the metals from groups VIII and VIb onto the selected support.

After introducing the metals from groups VIII and VIb, and optional shaping of the catalyst, it undergoes an activation treatment. This treatment is generally intended to transform the molecular precursors of the metals into the oxide phase. In this case it is an oxidizing treatment, but simple drying of the catalyst may also be carried out. In the case of an oxidizing treatment, also known as calcining, this is generally carried out in air or in diluted oxygen, and the treatment temperature is generally in the range 200° C. to 550° C., preferably in the range 300° C. to 500° C.

After calcining, the metals deposited on the support are in the oxide form. In the case of nickel and molybdenum, the metals are principally in the form of MoO₃ and NiO. Before contact with the feed to be treated, the catalysts undergo a sulphurization step. Sulphurization is preferably carried out in a sulphoreducing medium, i.e. in the presence of H₂S and hydrogen, in order to transform the metallic oxides into sulphides such as, for example, MoS₂ and Ni₃S₂. Sulphurization is carried out by injecting a stream containing H₂S and hydrogen over the catalyst, or a sulphur-containing compound which is capable of decomposing into H₂S in the presence of the catalyst and hydrogen. Polysulphides such as dimethyldisulphide (DMDS) are H₂S precursors which are in routine use for catalyst sulphurization. The temperature is adjusted so that the H₂S reacts with the metallic oxides to form metallic sulphides. This sulphurization may be carried out in situ or ex situ (inside or outside the reactor) as regards the demercaptanization reactor at temperatures in the range 200° C. to 600° C. and more preferably in the range 300° C. to 500° C.

In step a) for addition of mercaptans to the olefins, the feed to be treated is brought into contact with the catalyst in the sulphide form. The demercaptization reactions of the invention are characterized by an elimination of the mercaptans involving the olefins:

-   -   either by direct addition to the double bond to produce         sulphides with a higher boiling point;     -   or by a hydrogenolysing pathway: hydrogen (if it is present) in         the reactor will produce H₂S by contact with a mercaptan which         will add directly onto the double bond of an olefin to form a         heavier mercaptan, i.e. with a higher boiling point. This         pathway is, however, a minor pathway under the preferred         conditions of the reaction.

Similarly, in the case of the presence of H₂S in the feed, it is converted into mercaptan (which could itself be converted) by addition onto the olefins by means of the catalyst under the selected conditions. This H₂S may derive from recycle gas or from makeup hydrogen which might contain some impurities. This H₂S may also occasionally be dissolved in the liquid feed.

This step may be carried out without the addition of hydrogen to the reactor, but preferably it is injected with the feed in order to maintain a hydrogenating surface condition at the catalyst which is appropriate for high demercaptization conversions. Typically, step a) functions with a H₂/HC ratio in the range 0 to 25 Nm³ of hydrogen per m³ of feed, preferably in the range 0 to 10 Nm³ of hydrogen per m³ of feed, highly preferably in the range 0 to 5 Nm³ of hydrogen per m³ of feed, and still more preferably in the range 0.5 to 2 Nm³ of hydrogen per m³ of feed.

The whole of the feed is generally injected into the reactor inlet. However, it may in some cases be advantageous to inject a fraction or all of the feed between two consecutive catalytic beds placed in the reactor. This embodiment can in particular be used so that the reactor can continue to be operated if the reactor inlet becomes blocked by deposits of polymers, particles or gums present in the feed.

The gasoline to be treated is brought into contact with the catalyst at a temperature in the range 50° C. to 250° C., preferably in the range 80° C. to 220° C., and more preferably in the range 90° C. to 200° C., with a liquid hourly space velocity (LHSV) in the range 0.5 h⁻¹ to 10 h⁻¹, the unit for the liquid hourly space velocity being a litre of feed per litre of catalyst per hour (L/L·h). The pressure is in the range 0.4 MPa to 5 MPa, preferably in the range 0.6 to 2 MPa and still more preferably in the range 0.6 to 1 MPa.

At the end of step a), the gasoline treated under the conditions mentioned above has a reduced mercaptans content. Generally, the gasoline produced contains less than 50 ppm by weight of mercaptans, preferably less than 10 ppm by weight. More than 80%, or even more than 90% of the light sulphur-containing compounds with a boiling point below that of thiophene (84° C.) is generally converted. The olefins are not hydrogenated or are only very slightly hydrogenated, which means that a good octane number can be maintained at the outlet from step a). As a general rule, the degree of hydrogenation of the olefins is less than 2%.

Step b) for Treatment of the Gasoline Obtained from Step a).

The second step of the process of the invention involves a distillation column incorporating a catalytic reaction section. In said column, distillation of the gasoline obtained from step a) into at least two cuts is carried out, namely into:

-   -   a cut termed a “light” cut which contains a reduced sulphur         gasoline with a boiling point range which typically extends from         approximately the initial point of the feed at the inlet to the         catalytic column to an end point which is generally in the range         60° C. to 100° C.; and     -   a cut termed a “heavy” cut with a boiling point range which         typically extends from approximately the end point of the light         gasoline cut to approximately the end point of the feed for the         catalytic column and which comprises the majority of the heavy         sulphur-containing compounds, i.e. sulphur-containing compounds         with a boiling point which is higher than the cut point selected         for the operation of the column.

The catalytic distillation column comprises a reaction section comprising a catalytic bed which is intended to carry out at least one reaction for the thioetherification of mercaptans which have not reacted with the olefins during step a), selective hydrogenation of the diolefins of the gasoline, and possibly isomerization of the olefins. To this end, the reaction section is disposed in the distillation column such that the “light” gasoline which distils towards the top of the column is brought into contact with the catalytic bed.

The thioetherification reaction of step b) consists of reacting the residual mercaptans with the diolefins contained in the gasoline obtained from step a) to form thioethers with a boiling point which is higher than that of the residual mercaptans such that they are entrained in the “heavy” cut at the bottom of the catalytic column.

The function of the catalytic column involves the simultaneous presence of two phases in the reaction zone, namely a liquid phase and a vapour phase which includes the major portion of the hydrogen and light hydrocarbons.

As in any distillation, there is a temperature gradient in the system such that the lower end of the column comprises compounds with a boiling point which is higher than that of the upper end of the column. Distillation can be used to separate the compounds present in the feed by boiling point difference.

The heat of reaction which could be generated in the catalytic column is evacuated by evaporation of the mixture on the distillation plate concerned. As a consequence, the thermal profile of the column is very stable and the catalytic reactions which occur on the bed located at the column head do not perturb its operation. Similarly, this stability of the thermal profile means that stable reaction kinetics can be obtained since they are isothermal over each separation stage.

Preferably, the catalyst used in the reaction section is also capable, by means of a makeup of hydrogen, of selectively hydrogenating the diolefins of the “light” cut to form the corresponding olefins. Optionally, the catalyst of the reaction section can also isomerize olefins with a double bond which is in the external position to an isomer with the double bond in the internal position.

In accordance with the invention, the catalyst employed in the reaction section comprises at least one metal from group VIII deposited on a porous support and may originally be in the form of small diameter extrudates or spheres. The catalyst has a structural shape which is adapted to catalytic distillation in order to act both as a catalytic agent to carry out the reactions, but also as a material transfer agent in order to have separation stages available throughout the length of the bed.

In a preferred embodiment, the metal may be selected from nickel and palladium. If the metal is palladium, it is preferably the only active metal in the catalyst. When the metal is nickel, the quantity by weight of metal from group VIII with respect to the total weight of catalyst, expressed as the oxide, is generally in the range 10% to 60%.

When the metal is palladium, the quantity by weight of metal from group VIII with respect to the total catalyst weight (% Pd metal) is generally in the range 0.1% to 2%.

The porous support for the catalyst of step b) may be selected from alumina, nickel aluminate, silica, silicon carbide, or a mixture of these oxides. Preferably, alumina is used, more preferably pure alumina.

A catalyst which is particularly suitable for carrying out the addition of residual mercaptans onto diolefins and selective hydrogenation of the diolefins comprises 40% to 60% by weight of nickel deposited on an alumina support.

The catalytic reaction involves hydrogen which is mixed with the light gasoline which distils towards the head of the column and the mixture is brought into contact with the catalyst as defined. In the reaction section, the hydrogen/diolefins molar ratio is generally in the range 1 to 10 mol/mol. However, it is preferable to operate in the presence of a slight excess of hydrogen with respect to the diolefins in order to avoid too much hydrogenation of the olefins and maintain a good octane number.

The operating pressure of the catalytic distillation column is generally in the range 0.4 to 5 MPa, preferably in the range 0.6 to 2 MPa and more preferably in the range 0.6 to 1 MPa. The temperature prevailing in the reaction zone is generally in the range 50° C. to 150° C., preferably in the range 80° C. to 130° C.

It should be noted that this pressure may be substantially the same (except for the pressure drop) as that prevailing in the reactor for step a). The hydrogen required in step b) is relatively small as it essentially acts to maintain the catalyst in a hydrogenating state and to hydrogenate only the diolefins to olefins. Controlling the pressure of the column may impose it on the set of the two steps. Thus, a recycle of the hydrogen recovered in the head drum of the column may optionally be carried out. The recycle hydrogen may, for example, be injected directly into the catalytic distillation column of step b), or it may be injected with makeup hydrogen from the distillation column or the reactor and also may be injected directly into the reactor for step a).

In accordance with a particular embodiment, the distillation column is configured to function as a depentanizer.

In accordance with another particular embodiment, the distillation column is configured to function as a dehexanizer.

In the context of the invention, it is also possible to use more than one catalytic bed in the reaction zone, for example two distinct catalytic beds, separated from each other by a gap.

BRIEF DESCRIPTION OF THE FIGURES

These and other aspects of the invention will be clarified in the detailed description of particular embodiments of the invention, made with reference to the figures in the drawings, in which:

FIG. 1 shows a first layout of the process of the invention;

FIG. 2 shows a second layout of the process of the invention.

The figures are not drawn to scale. In general, similar elements are denoted by identical references in the figures.

FIG. 1 shows a first layout of the process of the invention for the treatment of a gasoline feed primarily comprising olefins, diolefins and sulphur-containing compounds of the mercaptan type and thiophene family type with a view to providing a light gasoline fraction with a total sulphur content of less than 30 ppm (by weight), preferably less than 20 ppm (by weight), or even less than 10 ppm (by weight).

In accordance with the process, the gasoline feed to be treated is sent with an optional makeup of hydrogen to a demercaptanization reactor 2 by means of a feed line 1.

The reactor 2 comprises a catalytic section provided with a catalytic bed specifically selected to carry out selective addition of mercaptans to the olefins with a view to increasing their molecular weight.

The reactor is preferably a fixed catalytic bed reactor which operates in a three-phase or two-phase system with one of the phases (the catalyst) being solid.

The demercaptization catalyst of the invention comprises a support, at least one metal selected from group VIII and at least one metal selected from group VIb of the periodic classification of the elements. The % by weight, expressed as the oxide equivalent, of the metal selected from group VIII is generally in the range 1% to 30% with respect to the total catalyst weight. The % by weight, expressed as the oxide equivalent, of the metal selected from group VIb being in the range 1% to 30% with respect to the total catalyst weight.

Preferably, the metal from group VIII is selected from nickel and cobalt and the metal from group VIb is selected from molybdenum and tungsten.

The demercaptanization reactions are generally carried out at a temperature in the range 50° C. to 250° C., at a pressure in the range 0.4 to 5 MPa and at a HSV in the range 0.5 h⁻¹ to 10 h⁻¹.

All or a portion of the feed treated in the reactor 2 for weighting the mercaptans is then sent to the distillation column 4 via the line 3, comprising a reaction zone 5 equipped with a catalytic bed, the assembly also being designated by the term “catalytic column”. Although FIG. 1 shows a reaction zone 5 with a single catalytic bed, it is entirely possible to use more than one catalytic bed, the beds being disposed in succession in the column.

The catalytic column 4 is configured and operated so as to fractionate the gasoline feed treated in reactor 2 into two cuts (or fractions), namely a heavy cut and a light cut. Further, the catalytic bed is disposed in the upper portion of said column such that the light cut encounters the catalytic bed during fractionation. In the context of the invention, the upper portion of the column which includes the reaction zone 5 is used to carry out not only a reaction for weighting the light mercaptans which have not reacted in the reactor 2, but also a selective hydrogenation of the diolefins present in the light cut or even an isomerization of the olefins. To this end, the light cut is brought into contact, in the presence of hydrogen, with at least one catalyst for the reaction zone 5 which comprises a support and at least one metal from group VIII of the periodic classification of the elements.

As can be seen in FIG. 1, hydrogen is sent directly to the catalytic column 5 via a line 6 the injection point for which is located upstream of the reaction zone. However, it should be noted that the hydrogen may be mixed directly with the effluent from the outlet from the reactor 2 at the level of the line 3, before the effluent is transferred to the catalytic column, as indicated in dashed lines in FIG. 1.

Referring to FIG. 1, the light cut containing a sulphur-reduced gasoline, unreacted hydrogen and possibly a little hydrogen sulphide is withdrawn via the line 8. The cut is then partially condensed in order to separate the noncondensable compounds by passage through a heat exchanger 9 before being sent to a separator 11. This latter can be used to recover, via the line 14, a gas fraction essentially containing hydrogen and a treated gasoline a portion of which is sent to the gasoline pool via the line 13 and the other portion of which is recycled to the catalytic column 4 via the line 12 to provide it with a reflux. The hydrogen recovered in the overhead gas may be recycled with the aid of a compressor. This recycle may be sent to the hydrogen makeup for the column or the hydrogen makeup for the reactor. Preferably, the light cut is withdrawn several plates below the head of the catalytic distillation column 5.

FIG. 2 illustrates a second embodiment which differs from that of FIG. 1 by the supplemental presence of an intermediate side stream 15 which, for example, can be used to specifically recover a gasoline cut containing a substantial portion of the thiophene compounds which could act as a feed for a secondary hydrodesulphurization type treatment unit. This side stream may be withdrawn below the catalytic zone, as can be seen in FIG. 2, or indeed at an intermediate point at the middle of the catalytic bed.

It should also be emphasized that the heavy cut which is withdrawn from the bottom of the catalytic column via the line 7 may be treated in a hydrodesulphurization unit, with a view to transforming the sulphur-containing compounds into H₂S, and then combined with the light gasoline withdrawn from the head of the catalytic column.

EXAMPLE

50 cc of a NiMo 8/8 catalyst on a nickel aluminate support in the form of 2-4 mm spheres was charged into a fixed bed downflow reactor. It was initially sulphurized by injecting a feed of heptane containing 4% DMDS at a hydrogen flow rate of 500 NL/L over 4 h, at a HSV of 2 h⁻¹, at 350° C. and at 2.5 MPa. Under these conditions, the DMDS decomposed to form H₂S and allowed sulphurization of the catalyst to take place.

The feed used for the test was a FCC gasoline with an initial boiling point IP=2° C. and a final boiling point FP=208° C.

The operating conditions were as follows:

-   -   P=1.0 MPa     -   T=100° C.     -   HSV=3 h⁻¹     -   H₂/HC=2 NL/L

An analysis by sulphur-containing compound species provided the following:

Compounds Feed (ppm) Effluent (ppm) H2S 5 <0.1 RSH C1-C6 213 8 THIOPHENICS 336 365 THIOPHANES 11 18 BENZOTHIOPHENES 333 322 SULPHIDES AND RSH C6+ 50 268 Total 943 981 The distribution of the mercaptans by number of carbon atoms was as follows:

Compounds Feed (ppm) Effluent (ppm) Sum: C1SH-C3SH ppm 211 7 Sum: RSH ppm 213 8

It will be seen that under these conditions, a conversion of 96.7% was obtained for the C1 to C3 mercaptans. These mercaptans are the sulphur-containing compounds which are the most susceptible of being found in the light fraction of the gasoline after distillation, which would result in more than 25 ppm of mercaptans in the light fraction.

Chromatographic analysis of the feed and the effluent provided the following results for the hydrocarbon families:

Compounds Feed (%) Effluent (%) Paraffins 29.0 28.9 Olefins 50.0 49.8 Naphthenes 8.8 8.9 Aromatics 12.2 12.2 C5 diolefins 0.31 0.26

It will be seen that the olefins were almost unhydrogenated between the inlet and the outlet for the reactor 2. Thus, the octane number was not degraded.

In addition, the chromatographic method used allowed the C5 diolefins to be identified, which were withdrawn along with the olefins family, and which provided us with an idea of the hydrogenation of these highly unsaturated compounds which were the most susceptible of being found in the overhead cut. These diolefins were: isoprene, 1,3-cis-pentadiene and 1,3-trans-pentadiene. Their conversion was approximately 17% in the reactor.

Next, the effluent from the reactor was sent to a catalytic column 5 cm in diameter and 12 m in height. The upper portion of this column was charged with 3 m of a catalyst which contained 0.3% by weight of Pd on an alumina-based support.

The operating conditions were as follows:

-   -   overhead pressure: 0.9 MPa     -   mean temperature of catalytic bed: 130° C.     -   overhead product yield: 25% by weight     -   H₂ flow rate/feed flow rate: 2 N litre/litre

The following results were obtained for the light fraction recovered from the upper portion of the catalytic column:

-   -   Total RSH content: 1 ppm by weight     -   Total S content: 8 ppm by weight     -   Diolefins content: 100 ppm by weight

This second step meant that conversion of the mercaptans could be finalized by addition onto the diolefins and meant that less than 10 ppm by weight of sulphur could be recovered by means of a side stream withdrawal above the catalytic zone. 

1. A process for the treatment of a gasoline comprising diolefins, olefins and sulphur-containing compounds including mercaptans, said process comprising the following steps: a) carrying out a step for demercaptanization by addition of at least a portion of the mercaptans onto the olefins by bringing gasoline into contact with at least one first catalyst, at a temperature in the range 50° C. to 250° C., at a pressure in the range 0.4 to 5 MPa and with a liquid hourly space velocity (LHSV) in the range 0.5 to 10 h⁻¹, the first catalyst being in the sulphide form and comprising a first support, at least one metal selected from group VET and at least one metal selected from group VIb of the periodic classification of the elements, the % by weight, expressed as the oxide equivalent of the metal selected from group VIII with respect to the total catalyst weight, being in the range 1% to 30% by weight and the % by weight, expressed as the oxide equivalent of the metal selected from group VIb, being in the range 1% to 30% with respect to the total catalyst weight; b) carrying out a step for treating the gasoline obtained from step a) with hydrogen in a distillation column comprising at least one reaction zone comprising at least one second catalyst comprising a second support and at least one metal from group VIII, the conditions of step b) being selected such that the following operations are carried out simultaneously in said distillation column: I) distillation in order to separate the gasoline obtained from step a) into a light gasoline fraction which is depleted in sulphur-containing compounds and into a heavy gasoline fraction with a boiling point which is higher than that of the light gasoline and comprising the majority of the sulphur-containing compounds, the light gasoline fraction being evacuated at a point located above the reaction zone and the heavy gasoline fraction being evacuated at a point below the reaction zone; II) bringing a fraction of the gasoline obtained from step a) into contact with the second catalyst in order to carry out the following reactions: i) thioetherification, by addition of a portion of the mercaptans onto a portion of the diolefins to form thioethers; i) selective hydrogenation of a portion of the diolefins to olefins; and optionally iii) isomerization of the olefins.
 2. The process according to claim 1, in which step b) is carried out at a pressure in the range 0.4 to 5 MPa, preferably in the range 0.6 to 2 MPa, and at a temperature in the range 50° C. to 150° C., preferably in the range 80° C. to 130° C., in the reaction zone.
 3. The process according to claim 1, in which the first catalyst comprises nickel and molybdenum, with a % by weight of nickel with respect to the total catalyst weight, expressed as the oxide, in the range 1% to 30%, preferably in the range 4% to 12%, and a % by weight of molybdenum with respect to the total catalyst weight, expressed as the oxide, in the range 1% to 30%, preferably in the range 6% to 18%.
 4. The process according to claim 1, in which the second catalyst comprises nickel with a % by weight of nickel with respect to the total catalyst weight, expressed as the oxide, in the range 10% to 60%.
 5. The process according to claim 1, in which the second catalyst comprises palladium with a % by weight of palladium metal with respect to the total catalyst weight in the range 0.1% to 2%.
 6. The process according to claim 1, in which the H₂/HC ratio in step a) is in the range 0 to 25 Nm³ of hydrogen per m³ of feed, preferably in the range 0.5 to 2 Nm³ of hydrogen per m³ of feed.
 7. The process according to claim 1, in which the gasoline is a catalytically cracked gasoline or any cracked gasoline containing olefins, diolefins and mercaptans, pure or as a mixture with straight run gasolines.
 8. The process according to claim 1, in which the distillation column is configured to function as a depentanizer or dehexanizer.
 9. The process according to claim 1, in which an intermediate gasoline cut is withdrawn as a side stream.
 10. The process according to claim 1, in which the pressure employed in the distillation column of step b) is equal to the pressure employed in the reactor for step a) apart from the pressure drop in the hydraulic circuit.
 11. The process according to claim 1, in which a step for condensation and separation is carried out on the light gasoline in order to recover unconsumed hydrogen.
 12. The process according to claim 11, in which unconsumed hydrogen is recycled to step a) or b).
 13. The process according to claim 1, in which in step a), addition of the H₂S present in the gasoline to be treated onto the olefins is carried out concomitantly.
 14. The process according to claim 1, in which the heavy gasoline fraction obtained from step b) is treated in a hydrodesulphurization unit. 